1. Field of the Invention
The present invention relates to an electric power steering apparatus assisting steering force of a steering wheel with the rotating force of a motor.
2. Description of the Related Art
An electric power steering apparatus has been developed which provides a driver a comfortable steering feeling by driving a motor for assisting steering force on the basis of a detected result of a steering torque exerted on a steering wheel and by assisting force required for steering a vehicle with turning force of the motor.
Such conventional electric power steering apparatus has a problem that when the steering wheel is returning to the neutral position upon receipt of an input from a tire, the returning movement is hindered by a moment of inertia of a rotor of a motor and frictional resistance of a reduction gear coupled to an output shaft of the motor. Therefore, a detector for detecting a steering angle, e.g., a rotary sensor or stroke sensor, etc. is installed on a rack shaft, steering shaft or motor to overcome the problem by driving the motor with a current corresponding to the detected steering angle, and the steering wheel is returned to the neutral position by the torque of the motor.
According to the above-mentioned arrangement, however, the installation of the steering angle detector makes the power steering apparatus expensive.
Moreover, a power steering apparatus to solve such problems as the poor response due to the inertia of the motor, friction of the reduction gear disposed between the motor and the steering mechanism, and phase delay in an electric circuit by differentiating a detected signal of a torque sensor thereby to adjust the driving current of the motor is disclosed in Japanese Patent Application Laid-Open No. 61-115771 (115771-1986).
More specifically, in the above disclosure, by differentiating the detected signal of the torque sensor and adding it to a temporary control value obtained corresponding to the detected signal of the torque sensor, the steering mechanism is driven in correspondence to the steering torque detected by the torque sensor without a time delay.
According to the conventional apparatus, the relation between the torque and the temporary control value of the driving current of the motor is preliminarily set as a function. The temporary control value of the driving current for the motor is set from the detected signal of the torque sensor by use of the function, and the differentiated signal of the detected signal is added to the temporary control value to determine the final control value of the driving current. The driving current for the motor is controlled by the determined control value thereby to assist steering force. The detected signal to be differentiated is the same as the detected signal for determining the temporary control value of the driving current. A voltage of the detected signal for determining the temporary control value is corresponding to the detected value of the steering torque.
FIG. 3 is a graph showing an example of the relation between the steering torque and signal voltage, in which the vertical axis represents the signal voltage and the horizontal axis represents the steering torque. When the detected steering torque is out of the range necessary to assist steering force under the normal cruising condition, the responsive voltage is saturated. In other words, the range necessary to assist steering force under the normal cruising condition is an effective detection width of the steering torque. Since the signal to be differentiated is the same signal as the detected signal for determining the temporary control value of the driving current, the differentiating is significant within the range of the effective detection width.
As is mentioned above, in the conventional apparatus, the detected signal of the steering torque to be differentiated is the same as the detected signal to determine the temporary control value of the driving current, and the voltage signal corresponding to the steering torque is saturated when the steering torque is out of the effective detection width. Therefore, the differential value becomes always 0 even if the steering torque is out of the effective detection width, the differentiating is not effective and the responding efficiency of the steering mechanism worsens.